My fighters are taking longer now than before to train and more food to feed. I’m base level 19 and..

As you unlock higher tier troops and upgrade the camps to allow you to train more then the time and costs will go up. You can reduce them a little by recruiting the right heroes to station in the training buildings, and unlocking their skills. You could also consider changing class to trader which gives you a 30% reduction in troop food consumption.

However it will be hard to avoid the costs and time taken forever if you want to have a strong fighting force.

The game will not let you run out of resources due to troop consumption so you in the long run its better to do what you can to earn resource crates and speedups through rewards such as for the daily challenges, Clash of zones etc. These resources are safe and stored until you need them, and the speed ups will let you train troops quickly.